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I think PO4-3 means PO4 anion, which is negative 3 in ionic charge. Normally written PO4 3- -- I can't do this in the response, but the 4 would be a subscript and the 3- (in other words, 3 minus) would be a superscript.

PO4 anion = orthophosphate. So if the results are really as PO4 3- then it is reprted as orthophosphate.

But beware: most labs report phosphate as P -- sometimes abbreviated PO4-P. So if I get a result of (say) 10 mg/L of phosphate (PO4 3-), I might report it as 3.2 mg/L PO4-P. It doesn't matter which you do, in the sense that they mean the same thing., but both the lab and the data user have to crystal clear on how the number is reported.

That calculation is as follows: atomic weight of phosphorus is 31, oxygen is 16. So PO4 is (31 + (4 x 16) = 91. The phosphorus is therefore 31/95 as a fraction of the phosphate anion: so 10 mg/L phosphate = 10 x (31/95) = 3.2 mg/L PO4-P.

Most of the P when it leaves a wastewater treatment is made up of bacterial solids. The bacteria will readily give up P and the result would be more otho P. My practice is to use total P when dealing with effluent.